This invention relates to the fabrication of circuits on insulating substrates, and in particular to a method of removing encapsulating material from said circuits.
Micro-electronic circuits, where many small circuit elements are formed on an insulating substrate, have found increasing use in modern technology. In particular, hybrid integrated circuits, employing a combination of thin film elements and silicon integrated circuit chips bonded on a single substrate, have developed in size and complexity for uses such as in memories and microwave transmission systems. In order to protect the components, the circuit is often covered by an RTV (Room Temperature Vulcanizing) silicone rubber which cures by reaction with moisture in the air. One of the problems associated with this technology is the fact that some of such circuits require diagnosis subsequent to forming the encapsulating material thereon, and the circuits are often found defective. Since many of these circuits are expensive, it is not economical to discard them, and repair is desirable.
There are primarily two methods of repair available in the art. If a failed component can be located underneath the opaque encapsulant, selective mechanical removal can be effected such as by peeling off the encapsulant over the defective component. (See, for example, U.S. patent application of Mancke and Soos, Ser. No. 727,057.) This procedure may be difficult if the encapsulant does not possess sufficient cohesive strength and/or has very high adhesive forces. A high pressure water jet technique may also be used to remove the encapsulant and a defective silicon device in one operation. (See, e.g., U.S. Pat. No. 3,969,813 issued to Minetti and Strickland.) This operation is not generally practical for removal of a defective crossover from a cluster or for multiple device repairs.
The second method of removal involves complete chemical stripping of the encapsulant. A problem arises in using many of such stripping solutions when the circuit includes components raised from the surface of the substrate, as when semiconductor chips are bonded to the substrate with beam leads. Specifically, the encapsulant under the chip tends to swell, causing unacceptable stresses on the leads resulting in mechanical and electrical failures. A further problem with many solutions is an adverse effect on insulation resistance. Although milder solutions were found to decrease the swelling, they usually involved too long a time for removal to be practical in a commercial process. Furthermore, it was found that most solutions left a visible residue which could not be removed even after long exposures. This tended to cause subsequent repair bonding to be ineffective and also produce inordinately high leakage currents across the substrate surface.
It is therefore a primary object of the invention to remove the encapsulant from hybrid integrated circuits without leaving significant residue or causing swelling of the encapsulant which results in damage to the components of the circuit. It is a further object of the invention to effect such removal in a period of time which is commercially practicable.